This study will evaluate the feasibility, cosmetic results, complication rates, and local control rate of 3D-CRT confined to the region of the lumpectomy cavity for patients with Stage I and IIa (less than or equal to 3 cm) carcinoma of the breast (non-lobular histology) treated with APBI using 3D-CRT before lumpectomy. For selected patients with Stage I and II breast carcinoma, 3D-CRT delivered to the primary tumor preoperatively is technically reproducible with acceptable complication rates. Cosmetic results after partial breast irradiation before lumpectomy will be comparable to that obtained after whole breast external beam radiation therapy. The local tumor control rate in the breast after partial breast irradiation therapy followed by lumpectomy will be comparable to that of conventional external beam radiation therapy, with less inconvenience and potentially less cost to the patient, given the selection criteria which minimize the risk of clinically significant multicentric or extensive residual carcinoma following lumpectomy.